1. Field of the Invention
This invention relates generally to optical low-pass filters, and more particularly is directed to an optical low-pass filter for use in the optical system of a color video camera which is of a single tube-type or which uses a solid state imager or the like.
2. Description of the Prior Art
In a single tube-type color video camera having a stripe-shaped color separation filter, or in a color video camera having a solid state imager, a camera output is obtained by optically spatially sampling the image of an object projected onto the pick-up device. In such a color video camera, the resolution or fineness of a picture pattern which can be handled is dependent on the sampling frequency. If the projected image contains a spatial frequency component higher than the sampling frequency, a false color signal or moire pattern results. When an image of the object in the field of view of the camera is formed on the stripe-shaped color separation filter and contains a frequency component which is the same as the spatial frequency of the stripe-shaped color separation filter, such frequency component causes a false signal to be produced which results in a reproduced picture having a color inconsistent with that of the original object.
For the reasons given above, the image pick-up optical system of a color video camera needs an optical low-pass filter which limits or restricts the high spatial frequency component of the image of an object in the field of view of the camera.
Some existing optical low-pass filters employ the birefringence or double refraction of crystalline materials, such as, calcite, quartz and the like. Such optical low-pass filters employing the birefringence of crystalline materials are adapted to selectively decrease the resolution of the frequency component of the image which is the same as the spatial frequency of the stripe-shaped color separation filter, while maintaining the highest resolution of the image in respect to the frequency band thereof which is lower than the spatial frequency of the stripe-shaped color separation filter. Further, optical low-pass filters employing the birefringence of crystalline materials can have the optical low-pass filter characteristics thereof varied by selecting or changing the thickness of the crystalline material or plate that is used therefore. However, since the crystalline material is expensive and a solid state imager requires several crystal plates, optical low-pass filters employing the birefringence of crystalline materials are uneconomical and present difficulties in their manufacture so as to be unsuitable for mass production.
Therefore, it has been proposed to use a phase noise type optical low-pass filter formed of a plastic material or the like, for example, as described in the article entitled "Spatial-Frequency Filter for a Kell-Type Color Camera", by R. L. Townsend, in Applied Optics, November 1972, volume 11, number 11, pages 2463 to 2472. In view of the fact that autocorrelation of the pupil function in a focusing optical system provides a modulation transfer function (MTF) of such optical system, an optical low-pass filter of the phase noise type obtains a desired characteristic by positively imparting wave front aberration to the pupil function. For example, by inserting into the optical system a transparent substrate on which a stripe-shaped thin film provides a phase difference, the phase term of the pupil function is varied so as to impart an optical low-pass filter characteristic thereto.
An optical low-pass filter for use in the optical system of a color video camera has to satisfy the following conditions:
(1) the MTF must be high in the frequency range below the cut-off spatial frequency so as to provide high contrast; and
(2) the MTF must be low over a relatively wide range of frequencies extending upwardly from the cut-off spatial frequency so as to minimize the false signal.
However, conventional phase noise type optical low-pass filters cannot satisfy the above requirements for an optical low-pass filter to be used in the optical system of a color video camera. Therefore, persons employed by the assignee of the present application and having an obligation to assign thereto, have previously proposed, for example, as disclosed specifically in Japanese laid-open Patent Application No. 61-126532, published June 14, 1986, a phase noise type optical low-pass filter having an improved periodic structure and an MTF characteristic which substantially satisfies the above noted requirements. More particularly, in such previously proposed phase noise type optical low-pass filter having an improved periodic structure and MTF characteristic, the MTF is suitably high for frequencies in a range lower than the cut-off spatial frequency which is the lowest frequency at which the MTF is zero, and the MTF remains low over a relatively wide range of frequencies extending from the cut-off spatial frequency to the next higher frequency at which the MTF becomes zero a second time. However, the MTF rises sharply at the higher spatial frequencies, that is, frequencies greater than that at which the MTF becomes zero a second time.
Such sharply increasing MTF at the higher spatial frequencies causes problems for certain kinds of imagers and color separation filters. More particularly, in a color video camera with a zoom lens, a false signal is made conspicuous when the zoom lens is operated so that it is necessary to provide a phase noise type optical low-pass filter with a further improved MTF characteristic, particularly for the higher spatial frequencies.